This invention generally relates to apparatus for testing glass containers. More particularly, this invention relates to an apparatus for testing surface coatings on glass containers. Specifically, this invention relates to an apparatus for measuring the force required to move two glass containers relative to one another against an applied and known normally directed load.
Many glass containers are coated with a two-layer, transparent coating of a lubricious organic material over a metal oxide adhered to the glass surface. Such coatings tend to prevent glass containers from scratching one another when they are in contact. This scratch-preventative coating helps maintain the original high strength level of the glass containers. U.S. Pat. No. 3,323,889, the teachings of which are hereby incorporated by reference, teaches such a coating process and a technique for testing the coating. Basically, two coated glass containers should be capable of relative movement while in contact with one another without scratching while a normal load of about one hundred pounds is applied. The basic test procedure is sound, but has proven difficult to monitor in practice. An operator of the test must listen carefully to hear a scratch or must carefully examine each container after a test to see if any scratches were produced. In addition, no information relative to the coefficient of friction of various coatings could be obtained from these tests. The coefficient of friction can be of importance in mass handling situations such as bottle filling lines. We have invented a test apparatus which will measure the force required to move the glass containers relative to one another while a normal load is applied. With this force known, the coefficient of friction may be determined. In addition, when the coating under test fails and allows scratching, the force required to move the containers rises sharply, thus alerting a test operator to carefully examine that particular set of glass containers.